1. Field of the Invention
The present invention relates to an in-mold coating process for applying coatings to moldings made of sheet molding compounds.
2. Description of the Prior Art
SMC is widely used for high-volume moldings of large, rigid parts. Factors favoring this use have been fast cure and easy flow of the SMC, high strength and stiffness, smoothness, low shrinkage, dimensional stability, and relatively low cost. However, still further improvement in surface quality for exterior automotive parts is a major need.
Particularly troublesome are surface porosity, waviness and sinks. None of these are effectively filled or masked by normal painting operations. Therefore, it is customary to inspect all parts and recycle defective ones through handfilling, sanding, painting and inspection stations until acceptable appearance is obtained. Where no sink can be tolerated, it is common to mold a separate part to act as a skin and bond to it a second part with stiffening and fastening sections.
Other approaches have been to mold a gel coat on parts to hide substrate flaws. In principle, this is done either by coating the hot mold before a charge is molded, see U.S. Pat. No. 3,940,468 (copy enclosed), or by molding a gel coat on a previously-molded part, see Japanese application 34210/62 (partial translated copy enclosed).
None of the above described methods of gel coating other than the urethane, appears to have received significant testing in hot matched metal SMC molding for a variety of reasons. Probably, the major technical obstacle has been lack of coatings which adhere to SMC other than through co-cure. Thus, where such coatings are applied first, the coating must be cured enough to resist tear during flow of the SMC but still be undercured enough to cross-link with the SMC. Similarly, when the SMC is molded first, it is necessary to open the mold before the SMC is fully cured, which is generally impractical since thin sections of parts are cured long before thick sections are adequately hardened.
One way around the problems described has been the use of resin impregnated foils in place of gel coatings. Another method has been use of unimpregnated foils which act as a tiecoat into whose opposite sides first SMC and later a gel coat can bite. The outstanding benefit with both methods is the ability to obtain surfaces with printed patterns. However, the utility is limited by folding or tearing of foil on many nonflat parts.
Another method has been to bond the gel coat to the substrate through isocyanate reactivity using a two component gel coat. The two part systems require mixing control, see British Pat. No. 1,457,935.